A touch panel system is designed to estimate a position of a touch and to process a specific function when a finger or an object touches a test displayed on a screen or a specific position of the screen without using an input device such as a keyboard, a mouse, and the like. The touch panel system is employed in various apparatuses, such as mobile phones, PDA (Personal Digital Assistants, notebook computers, navigation devices, PMPs (Portable Media Players), portable gaming devices, and the like. There are several types of this touch panel system such as a resistive touch panel system, a surface acoustic wave touch panel system, an infrared shield touch panel system, an electromagnetic induction touch panel system, and a capacitive touch panel system.
The capacitive touch panel system is classified into a type of determining a touch input using a self-capacitance generated between a touching object and a sensing electrode without applying a specific driving signal thereto, and a type of determining a touch input using mutual-capacitance generated between a touching object and a plurality of sensing electrodes while applying a specific driving signal thereto. The touch panel system using the self-capacitance has a simple circuit and is easily practiced but multi-touch is not easily determined. On the other hand, the touch panel system using the mutual-capacitance is advantageous of determining multi-touch over the self-capacitance touch panel system, but it has a disadvantage of an increased thickness because of dual layer structure thereof.
Each of these touch panel system includes, in general, a window to which a user's finger or an object is touched directly, a sensing electrode of a preset pattern, and a controller analyzing an electric signal provided from the sensing electrode to determine a position where a touch occurs. Here, the sensing electrode includes a plurality of channels and each of the channels corresponds to a touch sensing area.
The touch panel system may determine a touched position within a touch sensing area based on an electric characteristic obtained by comparing a sensing signal transmitted from each of the channels of the sensing electrode and a reference signal.
In the touch panel system as described above, detection of a touch is achieved by comparing capacitance difference caused by the touch with a reference voltage of a constant level, used to determine whether a touch occurs, which is generated by a mixer in the touch panel system. However, the existing mixer may not produce the voltage of constant level precisely due to a high frequency noise components contained in the sensing signal.